Structural analysis of sheath folds and geochronology in the Cuonadong Dome, southern Tibet, China: New constraints on the timing of the South Tibetan detachment system and its relationship to North Himalayan Gneiss Domes

Structural analysis and new zircon U/Pb geochronology were performed in the Cuonadong Dome in order to test the hypothesis that the top-to-North deformation in the North Himalayan Gneiss Domes (NHGD) has been correlated with deformation along the South Tibetan detachment system (STDS). Structural analysis suggests that the X-axes of sheath folds at the different flanks of the dome are consistent, and show average a trend and plunge of 350 degrees and 18 degrees. Many shear sense indicators are recorded in the ductile shear zone during the top-to-north deformation, including the augen structure, sigma-type and delta-type rotating porphyroclast, S-C fabric and pressure shadow structure. The development of the sheath folds first provide a direct-field structural evidence that represents a deeper ductile manifestation of the STDS, and further suggests that the top-to-north deformation in the NHGD have been correlated with deformation along the STDS. The emplacement of the synkinematic granites at ca. 32 Ma exposed at the core of the sheath folds is also interpreted as the initiation of slip along the STDS.